"That was funny!"
Her prominent eyes swam with tears as she gasped for breath, staring at Ron. Utterly nonplussed, he looked around at the others, who were now laughing at the expression on Ron's face and at the ludicrously prolonged laughter of Luna Lovegood, who was rocking backwards and forwards, clutching her sides.
"Are you taking the mickey?" said Ron, frowning at her.
"Baboon's … backside!" she choked, holding her ribs. -Harry Potter and the Order of the Phoenix

The Urban Dictionary T-Shirt

To minimize someone by ridicule, mockery or derision, especially if the clueless subject does not realize he or she is the target of ridicule. British in origin and similar to "taking the piss" out of someone.

Dick: (with mock sincerity and theatrical enthusiasm) Dang it, George! You like totally RULE, dude! I can’t decide if you are like, a great president…or like, THE greatest president!

George: (with feigned modesty) Shucks, Dick…t’aint no big thang!

Laura: (to George) CLEARLY, you are as dumb as a box of hammers and I would like to kick you as far as I could kick a lemon meringue pie…as usual…Dick is TAKING THE MICKEY OUT OF YOU!